The practice of making morpholine plus DIGLYCOLAMINE.RTM. amine from diethylene glycol and ammonia over an amination catalyst is well-known in the art (see, for example, U.S. Pat. No. 3,151,112 (1964). The reaction is generally run in an aqueous environment. The indication is that better productivity could be obtained using an anhydrous feed, since water acts as a diluent and takes up space in the reactor. Whether run under aqueous or anhydrous feed conditions, there are two principal gaseous by-products, methane and carbon dioxide. Under aqueous conditions most of the CO.sub.2 is absorbed into the H.sub.2 O. Under anhydrous feed conditions, even though water is a reaction by-product (e.g. see Equations 1 and 2), the CO.sub.2 will readily react with the excess ammonia to form ammonium bicarbonate and/or ammonium carbamate, both of which in the absence of adequate water diluent, will form solids which precipitate downstream of the amination reactor, in the heat exchanger, etc., causing additional unit maintenance costs and engineering problems. It would be extremely advantageous if it were possible to operate a morpholine/DIGLYCOLAMINE.RTM. process using anhydrous diethylene glycol/ammonia feed in such a fashion that CO.sub.2 by-product formation was lowered to below, or close to, threshold levels (i.e.&lt;50 ppm). Such a condition might be achieved by converting said CO.sub.2 to methane (Equation 3), possibly by introducing a methanation catalyst into the process unit, downstream of the diethylene glycol amination step. Particularly useful would be a methanation catalyst that:
a) Lowers the CO.sub.2 concentration in the morpholine/DIGLYCOLAMINE.RTM. amine product effluent to threshold levels.
b) Is stable in the high pH, amine, environment of the morpholine/DIGLYCOLAMINE.RTM. product effluent.
c) Remains an active methanation catalyst in said environment for extended periods, particularly at the temperature of DEG amination (190.degree.-250.degree. C.).
d) Does not convert the desired morpholine and DIGLYCOLAMINE.RTM. to unwanted by-products, as a result of catalyzing secondary reactions in the crude product effluent. ##STR1##
There is art pertaining to hydrogenation/dehydrogenation catalysts. In U.S. Pat. No. 4,508,896 there is described a process for simultaneously producing a 2-(2-aminoalkoxy)alkanol compound and a morpholine compound by contacting oxydialkanol with ammonia in the presence of a hydrogenation/dehydrogenation catalyst. In that process a hydrogenation catalyst was used which comprised about 60-85 mole % nickel, 14-37 mole % copper and 1-6 mole % chromium calculated on an oxide-free basis. This work does not address the problem of a methanation catalyst, CO.sub.2 production and its effect in lowering productivity. In said U.S. Pat. No. 4,508,896 to Templeton the hydrogenation/dehydrogenation catalyst is operated at a temperature of about 190.degree. C. to 230.degree. C. and a pressure of about 700 psig to about 2200 psig.
No art has been found dealing with a provision for converting by-product CO.sub.2 to methane when said CO.sub.2 is present in an amination product effluent, particularly where the CO.sub.2 is present in a morpholine/DIGLYCOLAMINE.RTM. amine product mix resulting from the amination of diethylene glycol (DEG) with ammonia.
It is known that carbon dioxide can be converted into methane by reduction with four moles of hydrogen (Equation 3). This is called the "Sabatier reaction". The mechanism for the reaction is not clear and early researchers assumed the reaction occurred in stages, however Bardet and Trambouze reported in work in C. R. Hebd. Seances. Acad. Sci., Ser. C. 288 (1979), 101, that it appeared CO.sub.2 methanation had its own mechanism, distinct from CO methanation, and that CO.sub.2 methanation occurs faster and at lower temperatures than methanation of CO. Heterogeneous catalysts have proven best for this process, see "Carbon Dioxide Activation by Metal Complexes," by A. Behr, Chapter 4, p. 85 (1988).
The mechanism for hydrogenation of CO.sub.2 on nickel is discussed in an article titled, "Hydrogenation of CO.sub.2 on Group VIII Metals II. Kinetics and Mechanism of CO.sub.2 Hydrogenation on Nickel," G. D. Weatherbee, et al., J. Catal., 77, 460-472 (1982). The rate of CO.sub.2 hydrogenation was measured as a function of H.sub.2 and CO.sub.2 partial pressures at 500-600 K, 140 kPa and 30,000-90,000 h.sup.-1. The data indicated the rate of CO.sub.2 hydrogenation is moderately dependent on CO.sub.2 and H.sub.2 concentrations at low partial pressures but essentially concentration independent at high partial pressures. Under typical conditions CO was observed as a product of the reaction at levels determined by quasi-equilibrium between surface and gas phase CO species. Addition of CO to the reactants above this equilibrium level caused a significant decrease in the rate of CO.sub.2 hydrogenation apparently as a result of product inhibition.
These authors set forth the following table which in their view summarized the mechanism for CO.sub.2 hydrogenation and accounted for the observations of this study and other recent studies.
______________________________________ Proposed Sequence of Elementary Steps in CO.sub.2 Methanation.sup.a Reaction ______________________________________ H.sub.2 (g) + S ##STR2## 2HS CO(g) + 2S ##STR3## COS + OS COS ##STR4## CO(g) + S COS + ##STR5## CS + OS CS + HS ##STR6## CHS + S CHS + HS ##STR7## CH.sub.2S + S CH.sub.2S + HS ##STR8## CH.sub.3S + S CH.sub.3S + HS ##STR9## CH.sub.4S + S CH.sub.4S ##STR10## CH.sub.4 (g) + S OS + HS ##STR11## OHS + S OHS + HS ##STR12## H.sub.2 OS + S H.sub.12 OS ##STR13## H.sub.2 O(g) + S ______________________________________ .sup.a S refers to a surface site.
The authors also noted that kinetic studies of methanation in their laboratory provided evidence that there are several mechanistic regimes involving different rate controlling steps, the importance of which depends mainly on temperature.
The authors proposed that at moderate reaction temperatures (525.degree.-575.degree. K.), CO dissociation appeared to be rate controlling while hydrogenation of carbon apparently controlled at higher reaction temperatures (&gt;575.degree. K.). This suggested that CO and CO.sub.2 methanation rates on nickel are both controlled by CO dissociation at moderate reaction temperatures (525-575.degree. K.), a hypothesis consistent with the observation of nearly identical specific reaction rates in CO.sub.2 and CO methanation on Ni/SiO.sub.2. The authors further proposed that it is possible for the two reactions to follow similar paths and still have different rate determining steps.
CO.sub.2 Methanation catalysts are discussed in an article titled "Highly Active Catalysts For CO.sub.2 Methanation To Provide The Second Reactor Of Two Stage Process For High BTU SNG Synthesis", A. D. Tomsett, et al., Applied Catal. 26 391 (1986). This investigation was made in the context of identifying highly active catalysts suitable for the second of two stages in a process for conversion of syngas to high BTU substituted natural gas. It was found that supported Ni-La.sub.2 O.sub.3 -Ru catalysts are effective for CO.sub.2 methanation. These catalysts provided high activity and complete conversion of CO.sub.2 to methane at as low a temperature as 250.degree. C.
It was concluded that the high activity of the Ni-La.sub.2 O.sub.3 -Ru catalysts is mainly ascribable to the large number of active sites.
It is known that methanation reactions are the reverse of reforming reactions. The methanation reactions are exothermic and, therefore, since a methanator typically operates in the temperature range of 300.degree.-400.degree. C. the CO and CO.sub.2 at the inlet should be carefully monitored to avoid damage to catalyst and vessel, see Kirk-Othmer, Encyclopedia of Chem. Tech., 3rd Edition, Vol. 2, p. 494.
Related art includes an investigation discussed in an article titled "Hydrogenation of CO.sub.2 over Co/Cu/K Catalysts", H. Baussart, et al., J. Chem. Soc. Faraday Trans. I, 83, 1711 (1987). Here Co, K and Cu, as components of catalysts were investigated for the specific roles each played in the hydrogenation of CO.sub.2. The results indicated, inter alia that a steady state was reached after ca. 30 hours of activation. It was found that the presence of cobalt favored the formation of alkanes, and that the presence of potassium favored the formation of CO resulting from a reverse water-gas shift reaction (CO.sub.2 +H.sub.2 .fwdarw.CO+H.sub.2 O). Further, it appeared the presence of copper in Russell catalysts increased the activity and the selectivity for CH.sub.4 and reduced the number of products.
There is a report on results of investigations into potassium-promoted nickel catalysts by T. K. Campbell, et al., Applied Catal., 50, 189 (1989). It was found potassium did not increase higher hydrocarbon or olefin selectivity, but it changed the CH.sub.4 /CO product distribution. It was also found that potassium did not change CO.sub.2 methanation mechanism and the mechanism appeared to be quite similar to that for CO hydrogenation. This paper presented steady-state kinetic data for CO.sub.2 hydrogenation on Ni/SiO.sub.2 and Ni/SiO.sub.2 -Al.sub.2 O.sub.3 catalysts for a range of potassium concentrations. It was reported that hydrogenation reactions of CO.sub.2 and CO on potassium promoted nickel catalysts are similar:
The activities and selectivities depend significantly on the support.
The rate of methanation on Ni/SiO.sub.2 decreases exponentially for both reactions with potassium addition.
The rate of methanation on Ni/SiO.sub.2 -Al.sub.2 O.sub.3 initially increases for both reactions with potassium addition.
Preoxidized rhodium was studied as a catalyst for methanation of carbon monoxide in an article titled "Methanation of Carbon Dioxide over Preoxidized Rhodium", A. Amariglio, et al., J. Catal., 81 247 (1983). The authors confirmed that according to their experiments preoxidation of Rh caused a dramatic enhancement of its activity in the methanation of CO.sub.2. They estimated preoxidation allowed the rate to be increased by a factor of 100 at the lowest estimate. The authors also concluded that the deactivation, observed upon prolonged exposures to H.sub.2, must be ascribed to the depletion of the preincorporated oxygen.
There is some information available regarding the use of ruthenium in a system structured for methanation alone. In an article entitled "Carbon Dioxide and Carbon Monoxide Methanation by Supported Ruthenium Catalysts", J. Chem. Soc. Chem. Commun., 955 (1982) by Helen Ferkul, et al., there is a discussion of ruthenium carbonyl clusters as active catalysts for the methanation of both carbon monoxide and carbon dioxide. The various carbonyl clusters were placed on different supports, such as .lambda.-alumina, silica gel or Na-Y zeolite. Added solvents were removed under reduced pressure and the catalyst systems containing 1% by weight of metal were activated by heating to 200.degree. C. in a stream of hydrogen. Catalytic activities were determined by passing synthesis gas through the catalysts in the temperature range of 200.degree.-400.degree. C. and space velocity of 1000-3750.sup.h-1. The major hydrocarbon product was methane. Though traces of C.sub.2 -C.sub.4 hydrocarbons were also obtained, the yields of C.sub.2 -C.sub.6 hydrocarbons apparently observed by others under similar conditions were not observed. (See T. Okuhara, et al., J. Chem. Soc., Chem. Commun., 1981, 1114). It was found that the best yields were obtained with Ru.sub.3 (CO).sub.12 on alumina. Comparison with the well-known RuCl.sub.3.3H.sub.2 O- Al.sub.2 O.sub.3 system showed that it did not deactivate as fast.
Additional information regarding the use of catalysts for CO.sub.2 methanation is found in an article by C. L. Thomas, entitled "Catalytic Processes and proven Catalysts", Academic Press., p. 116. It is noted that the most common ones are nickel. Many of the properties of the nickel catalysts are discussed along with disadvantages of using them in this type reaction. It is noted that in addition to nickel, platinum and ruthenium are used for methanation. There is no discussion here of the ruthenium catalyst.
From the art available it is evident that there are many factors and multiple variables involved in the methanation of CO.sub.2, such that it would be difficult to identify a catalyst which would promote methanation at low temperatures (&lt;250.degree. C.) and high pH within a dual catalyst system where the conditions are geared toward a primary reaction, such as selective amination.
It would constitute a distinct advance in the art if a catalyst system were available whereby CO.sub.2 formation could be suppressed during morpholine/DGA amine processing by amination of DEG under anhydrous conditions. It has been discovered that not just any methanation catalyst could be used in this fashion because of certain critical factors, particularly those outlined supra.
Unexpectedly it has been discovered that a dual catalyst system comprising a nickel in conjunction with a supported ruthenium catalyst is very effective for selective amination of DEG to morpholine plus DIGLYCOLAMINE.RTM. amine while at the same time significantly suppressing the coproduction of CO.sub.2.